This invention relates in general to sensors for anti-lock brake systems, and in particular to a structure for mounting an acceleration sensor within an anti-lock brake system control module.
An anti-lock brake system (ABS) is often included as standard equipment on new vehicles. When actuated, the ABS is operative to control the operation of some or all of the vehicle wheel brakes. A typical ABS includes a plurality of solenoid valves mounted within a control valve body and connected to the vehicle hydraulic brake system. Usually, a separate hydraulic source, such as a motor driven pump, is included in the ABS for reapplying hydraulic pressure to the controlled wheel brakes during an ABS braking cycle. The pump is typically included within the control valve body while the pump motor is mounted upon the exterior of the control valve body.
An ABS further includes an electronic control module which has a microprocessor. The control module is electrically coupled to the pump motor, a plurality of solenoid coils associated with the solenoid valves and wheel speed sensors for monitoring the speed and deceleration of the controlled wheels. The control module is typically mounted upon the control valve body to form a compact unit which is often referred to as an ABS electro-hydraulic control unit.
During vehicle operation, the microprocessor in the ABS control module continuously receives speed signals from the wheel speed sensors. The microprocessor monitors the speed signals for potential wheel lock-up conditions. When the vehicle brakes are applied and the microprocessor senses an impending wheel lock-up condition, the microprocessor is operative to actuate the pump motor and selectively operate the solenoid valves in the control unit to cyclically relieve and reapply hydraulic pressure to the controlled wheel brakes. The hydraulic pressure applied to the controlled wheel brakes is adjusted by the operation of the solenoid valves to limit wheel slippage to a safe level while continuing to produce adequate brake torque to decelerate the vehicle as desired by the driver.
It is known to include an acceleration sensor, commonly known as a "g-sensor", in an ABS. Acceleration sensors measure the movement of a seismic mass in an acceleration field. A spring element converts the force due to acceleration into a measurable deflection. Acceleration sensors typically include a sensing element in the form of a pendulum beam, which combines the spring element and the seismic mass into one flexible structure. The pendulum beam has a fixed end and a movable end. An acceleration or deceleration causes a deflection of the movable end of the pendulum beam about the fixed end.
The g-sensor measures the longitudinal acceleration or deceleration of the vehicle and sends a corresponding signal to the microprocessor in the ABS control module. The acceleration or deceleration signal from the g-sensor is independent of the speed signals from the wheel speed sensors. This input is beneficial in situations such as skidding of all four wheels of a four-wheel drive vehicle, indicating a low or zero wheel speed signal while the vehicle may actually be moving. The g-sensor senses this motion and provides an input to the ABS.
One type of g-sensor commonly used in ABS is a capacitive g-sensor. Capacitive g-sensors measure the change in capacitance between two electrodes when the physical separation of the electrodes changes in response to applied acceleration. One electrode is a pendulum beam, usually formed from silicon. The fixed end of the silicon beam is typically bonded to a rugged ceramic substrate. The silicon beam acts as one parallel plate in a capacitor. A thin film electrode on the substrate provides a second capacitor plate. The g-sensor also usually includes an electronic conditioning circuit which converts the capacitance determined by the separation of the electrodes into an analog output voltage. Capacitive g-sensors provide low temperature sensitivity and a large signal-to-noise ratio.
An acceleration or deceleration causes a deflection of the silicon beam about its fixed end, which causes a change in an sensor capacitance. The change in capacitance causes a corresponding change in the voltage produced by the conditioning circuit. Thus, the g-sensor measures the longitudinal acceleration or deceleration of the vehicle and generates a corresponding analog voltage signal which is proportional to the acceleration. The sensor output voltage is applied to an analog sensor input port on the microprocessor in the ABS control module. The microprocessor determines a vehicle reference speed by numerically integrating the g-sensor signal for comparison to the signals generated by the wheel speed sensors.